Inactivating Potassium Current Research Articles

Cultured Giant Fiber Lobe of Squid Expresses Three Distinct Potassium Channel Activities in Selective Combinations

Neurons from the giant fiber lobe (GFL) of squid Loligo bleekeri were dissociated and cultured. The ionic currents were recorded using whole-cell patch clamp methods. The sodium current and the noninactivating potassium current like those elicited by the giant axon were among the currents expressed in axonal bulbs and bulblike structures upon dissociation. Meanwhile axonless cell bodies did not elicit such currents. Axonless cell bodies and some bulblike structures elicited two kinds of inactivating potassium currents, the slow- and the fast-inactivating current, which differed in their inactivation kinetics and pharmacology. Within 24 hr of plating, the current composition remained the same. While the noninactivating current was not sensitive to 4-aminopyridine, the two inactivating currents were sensitive, the slow-inactivating current being more sensitive. Selective combinations of the sodium current and the three potassium currents expressed in different structures of the acutely dissociated GFL could have resulted from cellular control of synthesis and transportation of the channel proteins to the somatic and the axonal membrane. The sodium current and the noninactivating potassium current could be recorded from some axonless cell bodies maintained in culture for over three days, indicating that the separation of the giant axon from its somata could result in the transportation of the channels normally expressed on the giant axon membrane to the somatic membrane.

A slowly inactivating potassium current in CA3 pyramidal cells of rat hippocampus in vitro

The time- and voltage-dependent properties of a slowly inactivating K+ current were investigated by using the single-electrode current- and voltage-clamp recording technique in CA3 hippocampal cells of organotypic slice cultures. After a period of prolonged hyperpolarization, the onset of action-potential discharge in response to depolarizing current injection was delayed by several seconds. The conductances underlying this delay were identified in voltage-clamp recordings. A biphasically decaying outward current was evoked when the membrane potential was stepped back to -60 mV after a 30 sec period of hyperpolarization. The fast component was identified as the previously described D-current and was blocked by 100 microM 4-aminopyridine (4-AP). The slow component, which we refer to as IK(slow), appeared to be mediated by K+ ions, because its reversal potential shifted in a Nernstian manner with changes in extracellular K+ concentration. It decayed with a time constant of 7.5 sec and required a hyperpolarizing prepulse below -95 mV for 5.5 sec for 50% recovery from inactivation. IK(slow) was found to be voltage-dependent, with 50% activation occurring at -65 mV and 50% steady-state inactivation occurring at -84 mV. It displayed minimal or no sensitivity to the K(+)-channel blockers 4-AP (0.1-5 mM), Cs+ (1 mM), tetraethylammonium (10-50 mM), Ba2+ (1 mM), dendrotoxin-alpha (5-10 microM), charybdotoxin (0.5-2.5 microM), or glibenclamide (5-10 microM) and was not affected by preventing increases in intracellular Ca2+ concentration with Ca2+ chelators. IK(slow) was reduced by activation of metabotropic glutamatergic and cholinergic receptors. In summary, the biophysical characteristics of IK(slow) suggest a role in determining discharge onset after a period of membrane hyperpolarization, and its modulation by G-protein-coupled receptors reveals an additional function for these receptors in the control of cellular excitability.

Calcium-dependent, slowly inactivating potassium currents in cultured neurons of rat neocortex.

Slowly inactivating outward currents were examined in neurons from rat anterior cortex dissociated at postnatal day 1 and recorded after 7-48 days in vitro by the use of whole-cell patch-clamp technique, in the presence of 0.5-0.8 microM tetrodotoxin (TTX). 50 microM carbachol and 1-5 mM CsCl2. Experiments were often carried out in the additional presence of 1-5 mM CsCl2, which blocks the anomalous, inwardly rectifying IQ, the fast Ca(2+)-dependent K+ current (IC), and 50 microM carbachol, which depresses the IM current. These currents were evoked by depolarizing steps to -40 +/- 5 mV from a conditioning hyperpolarization to -110 +/- 10 mV. Their sensitivity to elevation from 2.5 to 12.5 mM in extracellular K+ concentration, together with their sensitivity to 5-15 mM tetraethylammonium, suggests that they are mainly carried by K+ ions. Their activation and inactivation curves show that the threshold for activation is -65 mV, that their inactivation is achieved at -75 mV and that potentials more negative than -120 mV are needed to abolish it. The time-dependence of de-inactivation gives a maximal current amplitude for conditioning hyperpolarizations of 2 s and is best described by a monoexponential function with a time constant of 0.7 s. Slow transient K+ currents were depressed by low doses of 4-aminopyridine (30-100 microM), which indicates the occurrence of an ID-type component in the recorded K+ currents. No slowly declining K+ current was expressed when a recording solution containing 10 mM 1,2-bis (2-aminophenoxy)ethane-N,N,N'-N'-tetraacetic acid (BAPTA), instead of 1-5 mM BAPTA, was used. When recorded without Ca2+ chelator in the pipette, slowly declining K+ currents were blocked by bath-applied 40-50 microM BAPTA-aminoethoxy, revealing a large-amplitude, rapidly inactivating outward current. This residual component is insensitive to 50 microM 4-aminopyridine and may include a current more related to the IA-type. Our data provide evidence that, in cultured cortical neurons from rat, the expression of an ID-like K+ current is highly dependent on internal Ca2+ concentration.

Elimination of potassium channel expression by antisense oligonucleotides in a pituitary cell line.

The clonal rat pituitary cell line GH4C1 expresses the genes for several voltage-dependent potassium channels including Kv1.5 and Kv1.4. Dexamethasone, a glucocorticoid agonist, induces a slowly inactivating potassium current in these cells but does not alter the amplitude of a rapidly inactivating component of potassium current. We have found that the induction of the slowly inactivating current can be blocked by an antisense phosphorothioate deoxyoligonucleotide to the Kv1.5 mRNA sequence. In contrast, antisense deoxyoligonucleotides against Kv1.4 mRNA specifically decrease the expression of the dexamethasone-insensitive rapidly inactivating current. These results demonstrate the usefulness of antisense oligonucleotides in correlating potassium currents with specific potassium channel proteins in the cell types in which they are naturally expressed.

Position‐dependent expression of potassium currents by chick cochlear hair cells.

1. Potassium currents in chick cochlear hair cells were studied using whole-cell voltage clamp techniques. Cells were isolated from 200 microns-long segments of the apical half of the cochlea. In each segment, expression of potassium currents by cells positioned across the width ('inner-outer' hair cell axis) of the cochlea was examined. 2. A rapidly inactivating potassium current (IA) was found in some hair cells. At a membrane potential of -24 mV, IA activated to peak values within 7 +/- 1 ms and inactivated within 73 +/- 16 ms. The activation 'threshold' was around -50 mV and hyperpolarization more negative than -56 +/- 5 mV was required before significant removal of inactivation occurred (V 1/2 (half-inactivation potential) = -74 +/- 5 mV). The resting potential of cells with IA was -46 mV +/- 11 mV. This current was blocked by 4-aminopyridine with a Kd of 0.45 mM. 3. Cells that were isolated from the most apical tip of the cochlea expressed no IA. In areas more basal than 200 microns from the apex, the magnitude of IA correlated with cell morphology. In each area, the tallest hair cells (cells with the smallest ratio of apical surface diameter to length) had none of this current. Of the cells with IA, the shorter cells (larger ratio of apical surface diameter to length) had more of this current. 4. The magnitude of IA in a cell was dependent upon cross-cochlear position, and the relationship between IA and cell morphology was most probably a reflection of a differential distribution of cell shape across the cochlea. The tallest hair cells, occupying roughly the first 40% of the distance from the neural side of the basilar papilla, had no IA. Of the remaining cells, those nearer to the abneural edge expressed more IA, such that iso-magnitude lines ran approximately parallel to the long axis of the cochlea. 5. A delayed rectifier current (IK) and an inward rectifier current (IIR) were also differentially distributed among hair cells across the cochlea; however, their distribution differed from that of IA. IK and IIR were preferentially expressed by the taller hair cells, which were positioned nearer to the neural side of the cochlea. Ca(2+)-activated potassium current (IK(Ca)) did not vary systematically between cells of different shape or cross-cochlear position, and IK(Ca) could often be found in cells with IA.

Ion channels in a skeletal muscle cell line from a Duchenne muscular dystrophy patient.

A cell line (RCDMD), derived from a muscle biopsy taken from a 7-year-old patient with Duchenne muscular dystrophy (DMD), was established in vitro using conditioned media from the UCHT1 thyroid cell line as described elsewhere (Biochim Biophys Acta 1992; 1134:247-255). Unlike other cell lines established by the same procedure, RCDMD cells were highly refractory to transformation and the resulting cell line grew slowly with a doubling time of approximately 72 h. Further, cells continue to grow after more than 20 doublings and 15 passages. Some of the characteristics of the cell line include lack of reaction with antidystrophin antibodies and the presence of receptors for the dihydropyridine PN200-110 (Kd) = 0.3 +/- 0.05 nmol/L and Bmax = 1.06 +/- 0.03 pmol/mg protein) and for alpha-bungarotoxin (Kd = 1.02 +/- 0.17 nmol/L and Bmax = 4.2 +/- 0.37 pmol/mg protein). Patch clamped cells in the voltage clamp configuration lack ion currents when growing in complete medium with high serum, but they can be induced to differentiate by serum deprivation and addition of hormones and trace elements. After 5 days in differentiating medium, noninactivating, delayed rectifier potassium currents are seen. At day 12, A-type, inactivating potassium currents as well as transient inward currents are seen. In conditions in which sodium and potassium currents are absent, a very fast activating and fast inactivating calcium current was evident. The cell line offers the possibility of studying cellular mechanisms in the pathophysiology of DMD.

Reconstruction of ionic currents in a molluscan photoreceptor

Two-microelectrode voltage-clamp measurements were made to determine the kinetics and voltage dependence of ionic currents across the soma membrane of the Hermissenda type B photoreceptor. The voltage-dependent outward potassium currents, IA and ICa(2+)-K+, the inward voltage-dependent calcium current, ICa2+ and the light-induced current, IIgt, were then described with Hodgkin-Huxley-type equations. The fast-activating and inactivating potassium current, IA, was described by the equation; IA(t) = gA(max)(ma infinity[1-exp(-t/tau ma)])3 x (ha infinity [1-exp(-t/tau ha)] + exp(-t/tau ha)) (Vm-EK), where the parameters ma infinity, ha infinity, tau ma, and tau ha are functions of membrane potential, Vm, and ma infinity and ha infinity are steady-state activation and inactivation parameters. Similarly, the calcium-dependent outward potassium current, ICa(2+)-K+, was described by the equation, ICa(2+)-K+ (t) = gc(max)(mc infinity(VC)(1-exp[-t/tau mc (VC)]))pc (hc infinity(VC) [1-exp(-t/tau hc)] + exp(-t/tau hc(VC)])pc(VC-EK). In high external potassium, ICa(2+)-K+ could be measured in approximate isolation from other currents as a voltage-dependent inward tail current following a depolarizing command pulse from a holding potential of -60 mV. A voltage-dependent inward calcium current across the type B soma membrane, ICa2+, activated rapidly, showed little inactivation, and was described by the equation: ICa2+ = gCa(max) [1 + exp](-Vm-5)/7]-1 (Vm-ECa), where gCa(max) was 0.5 microS. The light-induced current with both fast and slow phases was described by: IIgt(t) = IIgt1 + IIgt2 + IIgt3, IIgti = gIgti [1-exp(- ton/tau mi)] exp(-ton/tau hi)(Vm-EIgti) (i = 1, 2). For i = 3, /Igt(t) = gigt3m33h3(Vm - Eigt3)exp(-ton/Ton) x exp(-tfoff/t Off). Based on these reconstructions of ionic currents, learning-induced enhancement of the long lasting depolarization (LLD) of the photoreceptor'slight response was shown to arise from progressive inactivation of /A, lca2+ -K+, and lCa2+.

Voltage-gated currents in muscle cells of Schistosoma mansoni.

Three morphologically distinct types of muscle fibres isolated from the platyhelminth Schistosoma mansoni have been studied with whole cell current- and voltage-clamp techniques. Fibres showed a marked time-dependent decrease in membrane resistance in response to depolarizing current injections. Voltage-clamp experiments revealed the presence of two distinct voltage-gated outward currents. The most prominent current is a slowly activating, slowly and incompletely inactivating potassium current similar to delayed rectifier currents which have been described in a variety of cell types from a variety of organisms. Also present is a faster activating, quickly and completely inactivating potassium current that shares functional characteristics with 'A'-currents. All three of the cell types studied possess a delayed rectifier current, but only two of the three types have 'A'-currents. Though depolarization with high K+ leads to contraction of the dispersed fibres, no voltage-gated inward currents could be detected by whole cell voltage-clamp under any of our conditions.

The inactivating potassium currents of hair cells isolated from the crista ampullaris of the frog.

1. A-type outward currents were studied in sensory hair cells isolated from the semicircular canals (SCC) of the leopard frog (Rana pipiens) with whole-cell voltage- and current-clamping techniques. 2. There appear to be two classes of A-type outward-conducting potassium channels based on steady-state, kinetic, pharmacological parameters, and reversal potential. 3. The two classes of A-type currents differ in their steady-state inactivation properties as well as in the kinetics of inactivation. The steady-state inactivation properties are such that a significant portion of the fast channels are available from near the resting potential. 4. The inactivating channels studied do not appear to be calcium dependent. 5. The A-channels in hair cells appear to subserve functions that are analogous to IA functions in neurons, that is, modulating spike latency and Q (the oscillatory damping function). The A-currents appear to temporally limit the hair cell voltage response to a current injection.

Effects of general anaesthetics on neuronal sodium and potassium channels

1. 1. The effects of clinical inhalation anaesthetics, such as halothane and methoxyflurane, and “model” anaesthetics, such as hydrocarbons and n-alkanols, on neuronal sodium and potassium channels are reviewed. 2. 2. Lipid-based mechanisms for the actions of anaesthetics on the gating parameters of squid axon sodium and delayed rectifier potassium currents are considered in conjunction with evidence of more specific effects in other preparations, notably a fast inactivating potassium current in Helix neurones and a voltage-gated sodium current in rat dorsal root ganglion neurones. 3. 3. The proconvulsant actions of some inhalation anaesthetics are discussed in relation to the induction of spontaneous firing of action potentials in the squid giant axon.

Effects of isoflurane on K+ and Ca2+ conductance in isolated smooth muscle cells of canine cerebral arteries.

Although isoflurane is a known cerebral vasodilator, the mechanism of isoflurane-induced vasodilation is not clear. The purpose of this study was to investigate the effects of 2.6% isoflurane (1.2 mM) on macroscopic calcium and potassium channel currents in voltage-clamped canine middle cerebral artery cells. Cells were dialyzed with K(+)-glutamate solution and superfused with Tyrode's solution for measurement of potassium current (n = 20). Stepwise depolarization from a holding potential of -60 mV to beyond -30 mV elicited an outward, slowly inactivating potassium current that was reduced 50% +/- 2% and 81% +/- 3% (mean +/- SEM) in the presence of 1 mM 4-aminopyridine and 30 mM tetraethylammonium, respectively. Calcium ionophore (A23187, 10 microM) increased the potassium current by 76% +/- 3%, suggesting calcium dependency. Isoflurane reduced the amplitude of the potassium current by 35% +/- 4%. Calcium current was measured in cells dialyzed with solution containing 130 mM Cs(+)-glutamate and superfused with solution containing 10 mM BaCl2 and 135 mM tetraethylammonium to pharmacologically isolate the calcium current (n = 13). Under these conditions, progressive depolarizing steps from -60 mV elicited an inward current that was maximally activated at +20 mV and essentially eliminated by 1 microM nifedipine. This current, resembling a long-lasting (L-type) Ca2+ channel current, was reduced 40% +/- 4% by isoflurane. The results of this study suggest that isoflurane acts directly at the vascular muscle membrane to suppress transmembrane calcium and potassium currents. The decrease in calcium current would cause vasodilation; however, the concomitant decrease in potassium current may partially antagonize the depressant effect of isoflurane mediated through calcium current reduction.(ABSTRACT TRUNCATED AT 250 WORDS)

A K+ current in Xenopus muscle cells which shows inactivation

The purpose of this study was to describe an inactivating potassium current that is present in Xenopus muscle cells in culture. Whole cell currents were recorded from muscle cells grown in culture for 16-30 h. The majority of muscle cells had an outward current which inactivated during a 100 ms maintained depolarization. The outward current was reduced by tetraethylammonium or the aminopyridines, but not completely eliminated by either agent. Depolarizing prepulses reduced both the peak current and the late component of the current. The results suggest that by one day in culture these skeletal muscle cells develop an inactivating outward potassium current.

Functional properties of a slowly inactivating potassium current in guinea pig dorsal lateral geniculate relay neurons

1. The time- and voltage-dependent properties of a slowly inactivating and depolarization-activated potassium current and the functional consequences of its activation was investigated with current and single-electrode voltage-clamp techniques applied to guinea pig dorsal lateral geniculate neurons maintained as a slice in vitro. 2. In current clamp, application of a step depolarization to near firing threshold resulted in a slowly rising membrane potential that took up to 10 s to reach steady state and firing threshold. In voltage clamp, step depolarization of the membrane potential to values positive to approximately -65 mV resulted in the rapid activation followed by slow inactivation of an outward current. In both cases the sudden depolarization was associated with a large increase in membrane conductance, which gradually lessened in parallel with the slow depolarization in current clamp or with the decrease in outward current in voltage clamp. 3. The time course of inactivation of the outward current, which we refer to as IAs, was well fitted by a two-exponential function with time constants of 96 and 2,255 ms, suggesting the presence of a fast and slow phase of inactivation. The activation threshold for IAs was about -65 to -60 mV, whereas inactivation was incomplete even at -50 mV, suggesting the presence of a substantial "window" current. The time course of removal of inactivation of IAs at -85 to -100 mV was well fitted by a single exponential function with time constant of 91 ms. 4. IAs appears to be mediated by K+. Increasing [K+]o from 2.5 to 10 mM resulted in a reduction in amplitude of IAs, whereas changing from 10 to 2.5 mM [K+]o enhanced this current. Intracellular injection of Cs+ resulted in an abolition of IAs, whereas extracellular application of Ba2+ resulted in a large decrease in the apparent input conductance but relatively little reduction of IAs. 5. Both phases of inactivation of the transient outward current were completely blocked by low doses (100 microM) of 4-aminopyridine (4-AP), but not by extracellular application of Cs+, tetraethylammonium (TEA), tetrodotoxin (TTX), or after block of transmembrane Ca2+ currents. Local application of 4-AP to neurons depolarized to near firing threshold resulted in depolarization associated with a decrease in apparent input conductance, thereby confirming the presence of a window current.4+ this bias against depolarizing inputs.(ABSTRACT TRUNCATED AT 400 WORDS)

Slowly inactivating potassium current in cultured bull‐frog primary afferent and sympathetic neurones.

1. Cultured bull-frog dorsal root ganglion cells were voltage clamped in the whole-cell configuration. The cells were superfused with a nominally calcium-free Ringer solution containing tetrodotoxin (3 microM), magnesium (10 mM), cobalt (1 mM), barium (2 mM), 4-aminopyridine (3 mM) and caesium (2 mM). 2. Step depolarizations (10-40 mV, 100-300 ms) from a holding potential close to the rest (typically -70 mV) evoked an outward current (IK) followed by an outward tail current. The peak amplitude of the current was reduced to less than 10% by tetraethylammonium (30 mM). 3. IK developed to its peak in 200 ms at -30 mV. Tail currents reversed at potentials that changed according to the logarithm of the extracellular potassium concentrations. 4. Tail currents declined to the baseline according to an exponential function of time (tau congruent to 40 ms at -60 mV) and its reciprocal time constant increased e-fold with a 13 mV hyperpolarization. 5. The current inactivated during sustained (1-20 s) depolarizing pulses according to a single exponential function (tau congruent to 3 s). 6. The peak amplitude of IK at -30 mV was progressively increased as the holding potential was made more negative than -70 mV reaching the maximum with step depolarizations from -120 mV. Reversed phenomenon was observed as the holding potential was made less negative than -70 mV. 7. The removal of the steady-state inactivation occurred along with a single exponential function and the time constant was decreased from 70 ms at -70 mV to 10 ms at -120 mV. 8. It is suggested that a slowly inactivating potassium current which we called IK in amphibian sensory neurones could be a class of a 'delayed' rectifier potassium current. A potassium current with properties indistinguishable from those which have been described for the sensory IK also occurred in cultured bull-frog sympathetic neurones. 9. Forskolin (1-30 microM) and 1,9-dideoxy forskolin (10 microM) reduced the amplitude of IK by up to 85% but these actions were not mimicked by any of 8-bromo-cyclic AMP (1 mM), dibutyryl cyclic AMP (1 mM) and 3-isobutyl-1-methylxanthine (1 mM). A hydrophilic forskolin analogue, 7-O-hemisuccinyl-7-deacetyl forskolin (10 microM), was about one-tenth as potent as forskolin (10 microM).

Post‐inhibitory excitation and inhibition in layer V pyramidal neurones from cat sensorimotor cortex.

1. The effect of conditioning pre-pulses on repetitive firing evoked by intracellular current injection was studied in layer V pyramidal neurones in a brain slice preparation of cat sensorimotor cortex. Most cells displayed spike frequency adaptation (monotonic decline of firing rate to a tonic value) for several hundred milliseconds when depolarized from resting potential, but the cells differed in their response when pre-pulses to other potentials were employed. In one group of cells, the initial firing rate increased as the pre-pulse potential was made more negative (post-hyperpolarization excitation). Adaptation was abolished by depolarizing prepulses. In a second group, the initial firing rate decreased as the pre-pulse potential was made more negative (post-hyperpolarization inhibition). Hyperpolarizing pre-pulses caused the initial firing to fall below and accelerate to the tonic rate over a period of several seconds. A third group displayed a mixture of these two responses: the first three to seven interspike intervals became progressively shorter and subsequent intervals became progressively longer as the conditioning pre-pulse was made more negative (post-hyperpolarization mixed response). 2. Cells were filled with horseradish peroxidase or biocytin after the effect of pre-pulses was determined. All cells whose firing patterns were altered by pre-pulses were large layer V pyramidal neurones. Cells showing post-hyperpolarization excitation or a mixed response had tap root dendrites, fewer spines on the apical dendrite and larger soma diameters than cells showing post-hyperpolarization inhibition. 3. Other electrophysiological parameters varied systematically with the response to conditioning pre-pulses. Both the mean action potential duration and the input resistance of cells showing post-hyperpolarization excitation were about half the values measured in cells showing post-hyperpolarization inhibition. Values were intermediate in cells showing a post-hyperpolarization mixed response. The after-hyperpolarization following a single evoked action potential was 20% briefer in cells showing post-hyperpolarization excitation compared to those showing inhibition. 4. Membrane current measured during voltage clamp suggested that two ionic mechanisms accounted for the three response patterns. Post-hyperpolarization excitation was caused by deactivation of the inward rectifier current (Ih). Selective reduction of Ih with extracellular caesium diminished post-hyperpolarization excitation, whereas blockade of calcium influx had no effect. Post-hyperpolarization inhibition was caused by enhanced activation of a slowly inactivating potassium current. Selective reduction of this current with 4-aminopyridine diminished the post-hyperpolarization inhibition. 5. Chord conductances underlying both Ih and the slow-transient potassium current were measured and divided by leakage conductance to control for differences in cell size.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotonin enhances excitability in neostriatal neurons by reducing voltage-dependent potassium currents

The physiological effects of serotonin (5-HT) on rat neostriatal neurons were investigated using current-clamp techniques in neostriatal slices and voltage-clamp techniques in acutely dissociated adult neostriatal neurons. In most neurons (35/51), bath-applied 5-HT (10–60 gmM) decreased the first spike latency and increased the evoked firing frequency. Membrane input resistance was also increased in most neurons (35/35) but could not explain the enhanced responsiveness. Tetrodotoxin, at concentrations sufficient to block spike production, did not block the ability of 5-HT to enhance the slow ramp-like voltage trajectory produced by depolarizing current injection. The role of potassium currents in the 5-HT effect was examined using whole cell voltage-clamp; in 6 of 9 neurons, 5-HT reversibly decreased inactivating potassium currents activated by depolarization. These experiments suggest that 5-HT's effect on the ramp trajectory may be mediated by a reduction of potassium currents activated by sub-threshold depolarization.

Temporal integration by a slowly inactivating K+ current in hippocampal neurons.

A central aspect of neuronal function is how each nerve cell translated synaptic input into a sequence of action potentials that carry information along the axon, coded as spike frequency. When transduction from a graded depolarizing input to spikes is studied by injecting a depolarizing current, there is often a remarkably long delay to the first action potential, both in mammalian and molluscan neurons. Here, I report that the delayed excitation in rat hippocampal neurons is due to a slowly inactivating potassium current, ID. ID co-exists with other voltage-gated K+ currents, including a fast A current and a slow delayed rectifier current. As ID activates in the subthreshold range, and takes tens of seconds to recover from inactivation, it enables the cell to integrate separate depolarizing inputs over long times. ID also makes the encoding properties of the cell exceedingly sensitive to the prevailing membrane potential.

The influence of capsaicin on membrane currents in dorsal root ganglion neurones of guinea-pig and chicken

The effect of capsaicin on voltage-dependent membrane currents of isolated dorsal root ganglia (DRG) neurones of guinea-pig and chicken were investigated by the voltage-clamp technique and intracellular perfusion. In both species, administration of capsaicin (3 X 10(-5) M) to the outer surface of the cell membrane reduced the amplitude and accelerated the inactivation of the fast inactivating potassium current. In contrast, 3,4-diaminopyridine (3,4-DAP) reduced the fast potassium current without affecting the inactivation. Combined application of capsaicin and 3,4-DAP was more effective than either drug alone. The slow potassium current was diminished by capsaicin but not affected by 3,4-DAP. Capsaicin (3 X 10(-5) M) applied to the internal surface of the membrane had little effect on the fast outward current but primarily decreased the amplitude of the slow potassium current. Two subpopulations of sodium currents could be demonstrated in guinea-pig neurones according to their tetrodotoxin (TTX) sensitivity. In type I neurones the sodium current was completely blocked by TTX; type II neurones exhibited a TTX-sensitive as well as a TTX-resistant inward current. Capsaicin (3 X 10(-5) M) applied externally reduced the maximal amplitude of both current components. The time course of inactivation was delayed only in the TTX-resistant sodium current. The effect of capsaicin on Na-currents of DRG neurones was similar in guinea-pigs and chicken. In DRG neurones of chicken, only TTX-sensitive currents were observed. In both species the steady-state inactivation of the sodium currents was shifted by capsaicin to more negative potentials.

Voltage-dependent currents in neurones of the nuclei of the solitary tract of rat brainstem slices

Neurones within the ventral and ventrolateral nuclei of the solitary tract were analyzed under single-electrode current- and voltage-clamp conditions in rat brainstem slices. We present direct and indirect evidence for the existence of five different sorts of membrane currents: a tetrodotoxin-sensitive sodium current, a tetrodotoxin-resistant calcium current, a calcium-dependent potassium current, a non-inactivating potassium current which is inhibited by muscarine, an inactivating potassium current, which is inhibited by 4-aminopyridine. These membrane properties do not produce spontaneous bursting in these neurones. Assuming that neurones with such properties belong to the respiratory network, we discuss how conductances of this type may be involved in mechanisms regulating central respiratory activity.

Serotonin and cyclic adenosine 3':5'-monophosphate modulate the potassium current in tail sensory neurons in the pleural ganglion of Aplysia

Tail sensory neurons in the pleural ganglion that mediate the afferent portion of the tail withdrawal reflex in Aplysia californica undergo heterosynaptic facilitation of transmitter release during sensitization. As in the siphon sensory neurons, the transmitter serotonin produces facilitation and also elicits a slow, decreased conductance excitatory postsynaptic potential (EPSP) in these neurons. Using voltage clamp and biochemical analyses, we have found that the slow EPSP in the pleural sensory neurons is due to a decrease in a potassium conductance identical to the S potassium current characterized in siphon sensory neurons. Like the S current, the current modulated by serotonin in the pleural sensory neurons is a non-inactivating potassium current, and it contributes to both the resting and action potentials. The current reverses in 120 mM external K+ at -20 mV, close to the predicted Nernst equilibrium potential. Intracellular cesium blocks the serotonin response, but the current is not blocked by equimolar substitution of barium for calcium, nor by 50 mM tetraethylammonium chloride. The effect of serotonin is cAMP dependent, since serotonin elevates cAMP and both cAMP injection and forskolin mimic the serotonin response. These results indicate that the mechanism associated with sensitization of the siphon-gill withdrawal reflex, a slow decreased potassium conductance, is also a component of the neuronal circuitry underlying modulation of another reflex, the tail withdrawal reflex. Therefore, two distinct populations of neurons subserving similar behavioral functions have related biophysical and biochemical properties.